IIT Madras & University Of Nairobi Researchers Use Metamaterials To Enhance Detection Resolution Of Defects By Guided Wave Ultrasound

Guided Ultrasound Waves can be used for remote testing of engineering structures like buildings, pipelines and rails to prevent catastrophic failures that can occur due to corrosion, impact and strain.

Many engineering structures including buildings, pipelines and rails, among others, require periodic testing to prevent catastrophic failures that can occur due to corrosion, impact, and strain. High-frequency sound waves that travel in the bulk (bulk ultrasound) are widely used for non-invasive and non-destructive testing of structural materials. 

However, conventional bulk ultrasonic inspection is tedious and time-consuming, as it involves point-by-point assessment of structures, and this is especially challenging in large-scale assets. Although ultrasound-guided by features such as plates, bars, wires, pipes etc. is an attractive alternative, such ‘guided ultrasound’ is limited by resolution due to the relatively lower frequencies used.

In breakthrough research addressing this pressing challenge, Indian Institute of Technology Madras and University of Nairobi researchers have used metamaterials to improve detection of defects in large structures by guided wave ultrasound. 

The results of this collaborative work have recently been published in the international reputed peer-reviewed journal AIP Advances. The paper has been co-authored by Prof Prabhu Rajagopal, Department of Mechanical Engineering, IIT Madras, and his collaborators at the University of Nairobi – Dr Michael Gatari, and John Birir, a PhD student they jointly guide as part of a ‘Joint Degree Programme.’

“This work has much promise for remote inspection in industry and biomedicine,” the researchers write in their recent paper. With this knowledge, the team is expanding the concept to detect different sizes and geometries of defects.         

Elaborating on this Research, Prof Prabhu Rajagopal, Dept of Mechanical Engineering, IIT Madras, said, “The use of ultrasound scans in medical diagnostics is well-known and the principle remains the same for structural monitoring.”

Adding on, Prof. Prabhu Rajagopal said, “In conventional bulk ultrasound-based testing, the sound waves are sent into the sample, say pipe or pillar, perpendicular to the item, and a detector calculates the time interval between the transmission and reception of the sound waves that are either transmitted or reflected.  Sound waves travel at a uniform speed if the object is defect-free, but defects impede or deflect sound waves, which results in delays in reception.” 

Conventional ultrasound-based testing must be made at multiple regions of the test material and is therefore cumbersome to be used with large objects such as train tracks, oil-pipelines and reinforcing structures of tall buildings, among others

This is where Guided Wave Testing (GWT) helps. In GWT, the sound waves are sent along the length of the structure rather than into the structure.  This allows the waves to travel long distances. GWT has poorer resolution than conventional ultrasound-based testing due to diffraction limitations. Thus, guided waves are only a long-range screening tool and must be used in conjunction with a testing tool with better resolution for accurate detection of defects.

Prof Rajagopal and his research team used metamaterials to improve the resolution of guided ultrasound waves. “Metamaterials are artificially crafted materials with unique internal microstructures that give them properties not found in nature, explains Dr Rajagopal. The constituent artificial units of the metamaterial can be tailored in shape, size, and interatomic interaction, to exhibit unusual properties,” explained Prof. Rajagopal.

Acoustic metamaterials are useful in manipulating sound waves. The Researchers used a metamaterial structure consisting of a series of periodically arranged channels. With proper selection of channel size, length, and periodicity of the metamaterial, the evanescent waves arising from scattering by a defect can be magnified by a process called Fabry–Pérot resonance. Resonance is a phenomenon in which a wave, in this case, the ultrasound wave, is amplified due to a match in frequency between the wave and the frequency generated by the metamaterial.

The researchers used an aluminium bar with side-drilled holes as the test sample. The holes were the defects that had to be detected.  The specially fabricated metamaterial was placed just above the holes.  The transmitter was placed at the end farthest from the holes, and the receiver, close to the holes.  The metamaterial resulted in detection of the holes with improved resolution down to 1/72 of the operating wavelength, which is the highest reported worldwide, for the ultrasonic domain.  This level of resolution makes GWT comparable to bulk ultrasonic testing with the added advantage of longer range. Moreover, with skilful use of frequency, this approach also offers the tantalizing prospect of a guided ultrasonic microscope, with radical benefits for remote diagnostics.

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